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A system has been developed at the Mt John University Observatory to enable relative radial velocities of solar-type stars to be obtained with a characteristic random error of 55 m/s. The high radial-velocity precision has been achieved by interfacing a single optical fibre feed between the telescope and spectrograph, which has enabled the spectrograph to be mounted in a thermally and mechanically stable configuration and has virtually eliminated guiding errors.
Using this system, a programme of observation of 29 solar-type stars and 10 giant International Astronomical Union radial-velocity standard stars was carried out over 2.5 years with a view to the detection of low-mass companions to the dwarf stars. One star, HR3220, turned out to have a previously-undiscovered stellar companion but no dwarfs showed obvious radial-velocity variability suggesting the presence of sub-stellar companions, although β Hyi showed a possible variation. This is despite the programme's sensitivity to the discovery of companions of mass 20 M₄ or greater in orbits of periods less than about 8 years (and larger masses in longer period orbits). In contrast, at least half the giant 'standard' stars were variable in radial velocity. Four and possibly five of the giant standards are probably intrinsic (pulsating) red or yellow (Walker et al. 1989) variables. Two further standards, β Aqr and δ Sgr, showed long-period variability suggestive of companions of indeterminable but low mass.
The lack of brown dwarfs observed in this programme is consistent with the results of other recent surveys. High-mass brown dwarfs appear to be rare as companions to stars and are probably rare in the field as well. They are unlikely to contribute significantly to the local mass density. Low-mass brown dwarfs (or high-mass planets) seem to be rare in orbits closer than 10 AU but could yet be found to abound in wider orbits or in the field.